Lewis acid-catalyzed additions of .alpha.,.beta.-unsaturated ethers (enol ethers) to acetals have been known for a long time and date back to the work of Muller-Cunradi and Pieroh (see U.S. Pat. No. 2,165,962). Hoaglin and Hirsch [J.A.C.S. 71, 3468 et seq. (1949)] investigated this reaction further and broadened the possible applications, which Isler et al. likewise did in the nineteen fifties with respect to the synthesis of .beta.-carotene, crocetin dialdehyde, lycopene as well as .beta.-apocarotenoids [see Helv. Chim. Acta 39, 249 et seq. and 463 et seq. (1956), ibid. 42, 854 et seq. (1959) as well as U.S. Pat. Nos. 2,827,481 and 2,827,482]. Later, Mukaiyama [Angew. Chem. 89, 858 et seq. (1977) and Org. Reactions 28, 203 et seq. (1982)] extended the reaction by using the readily accessible trimethylsilyl enol ethers.
Reactions of ethenol alkyl ethers with cyclic acetals, e.g. ethylene acetals, are also known; they afford by addition cyclic compounds having two ring oxygen atoms, e.g. 1,4-dioxacycloheptane [see Mikhailov et al., Izv. Akad. Nauk. SSSR, Otd. Khim. Nauk. 1960, 1903 et seq./Chem. Abs. 55, 13409 f (1961) and German Patent 1,031,301/Chem. Abs. 54, 22712 d (1960)].
The first Lewis acid-catalyzed condensations of 1-alkoxy-1,3-dienes (dienol ethers) with .alpha.,.beta.-unsaturated acetals were reported by Nazarov and Krasnaya [J. Gen. Chem. USSR 28, 2477 et seq. (1958)] and by Makin [Pure & Appl. Chem. 47, 173 et seq. (1976), J. Gen. Chem. USSR 31, 3096 et seq. (1961) and 32, 3112 et seq. (1962)]. Here, the coupling of the acetal to the dienol ether takes place as far as can be seen exclusively at its .gamma.-position with the formation of a chain-lengthened .alpha.,.beta.-unsaturated acetal, which, however, in competition with the first acetal reacts with further dienol ether with the formation of a further, chain-lengthened .alpha.,.beta.-unsaturated acetal etc. [telomer formation; see also Chemla et al., Bull. Soc. Chim. Fr. 130, 200 et seq. (1993)]. For this reason such a condensation has been found not to be workable for synthetic purposes, especially for the synthesis of apocarotenoids [Isler et al., Adv. Org. Chem. 4, 115 et seq. (1963)].
Not only 1-alkoxy-1,3-dienes, but also trimethylsilyloxydienes [of the type CH.sub.2 .dbd.CH--CH.dbd.CH--OSi(CH.sub.3).sub.3 ] can be condensed with .alpha.,.beta.-unsaturated acetals in the presence of Lewis acid catalysts, as disclosed by Mukaiyama et al. in Chem. Lett. 1975, 319 et seq. In this coupling too the attack seems to take place exclusively at the terminal (.gamma.-) carbon atom of the diene system in order to form ".gamma.-products" [see Mukaiyama et al., Bull. Chem. Soc. Jap 50, 1161 et seq. (1977) and Japanese Patent Publication (Kokai) 36,645/1977/Chem. Abs. 87, 201825 t (1977)]. In contrast to the reaction with 1-alkoxy-1,3-dienes, in which an .alpha.,.beta.-unsaturated acetal results, the reaction of trimethylsilyloxydienes with acetals affords an aldehyde which does not react further with the diene (no telomer formation). Thereby, zinc bromide and many other Lewis acids are required as catalysts only in small amounts [Fleming (et al.), Tetr. Lett. 1979, 3209 et seq. and Chimia 34, 265 et seq. (1980) as well as Brownbridge, Synth. 1983, 85 et seq]. By using this method Mukaiyama et al. were able to synthesize vitamin A [see Kokai 36, 645/1977, Chem. Lett. 1975, 1201 et seq. and Bull. Chem. Soc. Japan 51, 2077 et seq. (1978)] and workers from Rhone-Poulenc developed new routes to carotenoids and vitamin A (see DOS 2,701,489 and A.E.C. Societe de Chimie Organique et Biologique No. 7824350).
The aforementioned Lewis acid-catalyzed condensation of a dienol ether with an .alpha.,.beta.-unsaturated acetal based on the works of Nazarov and Krasnaya, Makin as well as Chemla et al. would be a very valuable access to apocarotenals and bis-apocarotenals if the yield of the desired primary product of the type . . . CH.dbd.CH--CH(Oalkyl.sup.1)--CH.sub.2 --CH.dbd.CH--CH (Oalkyl.sup.1)(Oalkyl.sup.2) could be increased and the telomer formation could be suppressed. Thus, the desired polyene aldehyde of the type CH.dbd.CH--CH.dbd.CH--CH.dbd.CH--CHO could be obtained from this primary product by hydrolysis of the acetal group C(Oalkyl.sup.1)(Oalkyl.sup.2) and elimination of alkyl.sup.1 OH. In addition to the fact that in this reaction the formation of the double bond takes place under catalytic conditions, no phosphorus-, silicon- or sulphur-containing reagents are required.
Less known from the scientific literature is the coupling of an (.alpha.,.beta.-unsaturated ethylene acetal with a trimethylsilyloxydiene. Analogously to the "usual" dienol ether condensations with dialkyl acetals, the product results in moderate yields [see again Chem. Lett. 1975, 319 et seq. and Bull. Chem. Soc. Japan 50, 1161 et seq. (1977)] according to the equation: ##STR1##
U.S. Ser. No. 08/865,288 filed May 29, 1997 discloses a different process for the manufacture of polyene aldehydes in which a polyene O,O-dialkyl acetal is reacted with a 1-alkoxy-1,3-diene compound.
An object of the present invention is to manufacture chain-lengthened polyene aldehydes starting from polyene acetals while avoiding as far as possible the aforementioned disadvantages of the state of the art and replacing the Wittig, Horner or Julia reaction hitherto used for this purpose.